The present invention relates generally to a system for making impedance measurements of body tissue and, more particularly, to such a "bio-impedance" system which can measure the impedance of body tissue over a range of frequencies.
There are a number of commercially available systems which measure the impedance of body tissue by applying a current or voltage signal to the tissue and measuring the resulting voltage or current to determine the impedance of the tissue which is representative, for example, of fluid content within the tissue. As is well known, such systems can be used to measure blood flow, monitor heart functions, monitor fluid build-up within the lungs and perform similar functions for medically related research and patient treatment. All of the known commercially available systems apply a single frequency signal to make the impedance measurements.
There is a need for a bio-impedance measuring system which is capable of making measurements at any frequency within a range of frequencies to permit the study of electrical impedances of intracellular and extracellular fluid composition. Such measurements are essential for determining total body fluid in an intensive care unit, for trauma situations, for dialysis, for physical fitness evaluations (body fat), and similar applications. Problems encountered in developing multiple frequency bio-impedance measurement systems include the difficulty of maintaining low intensity constant drive current through body tissue loads which vary, and the ability to make consistent and accurate determinations of resistive and reactive impedance components at varying loads and different frequencies.
An impedance measuring system utilizing two different frequency signals is disclosed in U.S. Pat. No. 3,851,641. In this system, two different frequency electrical signals are alternately passed through body tissue with the resulting voltage signals being sensed. In-phase coherent detection is performed to remove out-of-phase signal components. The in-phase voltage signals are subtracted from one another to derive a measure of only the internal impedance of the body tissue.
Two different frequency signals are also used in U.S. Pat. No. 4,793,362 wherein change in body fluid is monitored by placing a pair of electrodes at spaced locations on a body, such as a wrist and diametrically opposed ankle, and passing two different high frequency signals through the body via the electrodes. Another pair of electrodes is place in the current path of the first pair of electrodes for measuring body impedance at the two frequencies. A microprocessor utilizes the impedance measurements in an empirical formula to calculate the change in fluid weight of the body.
Thus, while it is known to measure the impedance of body tissue utilizing a single frequency signal applied to the tissue or two frequency signals alternately applied to the tissue, the need for a multiple frequency bio-impedance measuring system which is capable of making impedance measurements at any frequency within a range of frequencies still remains.